Set-up with two subs and an integrated amp

+1 on the REL solution. REL integrates perfectly with the SPKON conection. If you get a preamp with an RCA sub-out later, REL can do that too. Wireless optional.

What ever you buy you need speaker high level inputs.

The REL has a high level input.  From your integrated speakers output, to the high level REL inputs via a SpeakOn cable with one end bare. The bare wire goes to you speaker terminals on the amp, as per instructions, Red right, positive, Yellow left, positive, black left or right Negative. 

Then out of your bass bins, high level outputs to you speakers, binding post..

Happy hunting

Regards

Another vote for REL. Through High Level Input, the sub integration with your main speakers is straight forward and seamless. There are plenty of YouTube video from REL on the high level connectivity.  

https://rel.net/tube-not-tube/

This is what you need:

https://www.crutchfield.com/S-BZsGXIya5Pj/p_543ADP12/Russound-ADP-1-2-Speaker-level-to-Line-level-Ad...

All you need is what we call a Line Out Converter or LOC. A simple circuit comprised of 2 resistors drops your Line Magnetic speaker level output to line level. In case you’re interested here is the best circuit diagram and electrical explanation on the internet.https://www.epanorama.net/circuits/speaker_to_line.html


Option 1, total cost $2, mod your amp to convert an unused line in into pre-out: scroll through the pictures until you get to the one showing the resistors inside the amp https://systems.audiogon.com/systems/8367 This is a picture of the actual mod. What this does is convert already existing RCA inputs that I did not need into a pre-amp out that I did need for my subs. Here you will see the same two resistors per channel indicated in the epanorama circuit diagram. This mod takes less than an hour all-in.

Option 2, total cost $20, but a LOC from Parts-Express. These are cheesy looking which is why I did my mod. You can pay a lot for an LOC but all you are getting is cosmetic, as inside they are all just the same two cheap resistors.

Option 3, same as Option 1 except you put the resistors inside the sub. If you search around for sub plate amp circuit diagrams you will find they all use this exact same circuit, two resistors, same cheap quality and everything.

Sorry for all the misleading advice. I got the same bad advice asking the exact same question a year ago. I did the research, found the answers, and did the mod. You can do it too. Then please share good information and not be telling people to spend thousands when $20 will do just fine.